In a typical cellular communication system, it is important to enable handover functionality. Handover is the process of transferring control over an ongoing connection between a (mobile) wireless communication device and the network providing the cellular communication system from one network node (the serving network node, providing a serving cell) to another network node (the target network node, providing a target cell). Handover is typically provided to accomplish a transparent service for the wireless communication device over a geographical area which extends beyond the coverage area of a single network node. Preferably, a handover should be performed without any (or with minimal) loss of data and without any (or with minimal) interruption in the communication of the ongoing connection.
Enabling of handover functionality typically comprises finding of a suitable target cell and ensuring (or making probable) that it is possible to sustain reliable communication with the found suitable target cell.
Candidate cells (provided by candidate network nodes) for finding of the suitable target cell are typically stored in neighbor lists, which may be stored at the serving network node or elsewhere in (or in association with) the network providing cellular communication system, as suitable.
To evaluate whether it is possible to sustain reliable communication with any of the candidate cells, the quality of a possible connection between the wireless communication device and the candidate cell are typically estimated before a decision to perform a handover takes place. Such estimation may typically be done based on downlink measurements carried out by the wireless communication device on reference signals transmitted by the candidate cells and reported to the serving network node.
In many typical cellular communication systems, each network node continuously transmits reference signals (e.g. pilot signals) that wireless communication devices in neighbor cells may use to estimate the quality of a possible connection with the network node. Examples of such reference signals comprise BCCH (broadcast control channel) in GSM (Global System for Mobile communication), CPICH (common pilot channel) in UMTS (Universal Mobile Telecommunication System), CRS (cell specific reference signal) in UMTS-LTE (UMTS, Long Term Evolution) and beacon signals in the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards.
Many emerging cellular communication systems may use advanced antenna systems to enable communication in narrow beams directed from the serving network node towards the wireless communication device, so-called beam-forming. Beam-forming may be used to enable high signal strength in the direction of the beam while the interference caused in other directions is kept low. Another advantage of beam-forming is that coverage of a network node may be increased.
In systems employing beam-forming, there is typically a need for a beam switch functionality, typically including switches between beams supported by the same network node as well as switches between beams supported by different network nodes (i.e. handover). In analogy with the established handover terminology, the beam before a beam switch is called the serving beam and the beam that will be used after the beam switch is called the target beam.
Throughout this disclosure, the term beam switch will be used to cover both the case where the serving beam and the target beam are supported by the same network node (i.e. when the beam switch does not involve a handover between network nodes, an intra-node beam switch) and the case where the serving beam and the target beam are supported by different network nodes (i.e. when the beam switch involves a handover between network nodes, an inter-node beam switch).
Transmitting reference signals in all beams continuously to enable measurements for beam switch decisions is not particularly efficient when a beam-forming system has a large number of narrow of beams. One reason is that, in some typical scenarios, only a few (or no) beams supported by a network node are active (e.g. used for a connection with a wireless communication device) and transmitting reference signals in the rest of the beams would only consume power, add interference and require extra hardware resources.
An alternative approach is to have only a subset of candidate beams transmitting reference signals, and only when it is probable that a beam switch (with or without a handover) is needed. Such reference signals may be termed mobility reference signals (MRS) and may, for example, have a similar physical structure to a secondary synchronization signal (SSS) as defined in UMTS-LTE or any other suitable signal structure.
To determine when it is probable that a beam switch is needed, the serving network node may use uplink measurements (typically making some assumptions regarding reciprocity) and/or channel quality reports from the wireless communication device regarding the connection. When the serving node determines that it is probable that a beam switch is needed it may trigger a mobility procedure, where the candidate beams transmit reference signals and the wireless communication device can perform and report measurements of the reference signals to the serving network node for the beam switch decision. The serving network node may inform the wireless communication device about the timing (e.g. start and/or end) and/or content (e.g. signal sequences) of the reference signals in association with triggering the mobility procedure.
Which beams to use as candidate beams may, for example, be based on content of a database (e.g. a mobility look-up table, LuT). Such a database may (in analogy with the neighbor cell list) comprise information regarding candidate beams for each serving beam and/or for each geographical location of the wireless communication device. The database may be formed and/or up-dated in any suitable way. For example, it may be based on system set-up parameters and/or on statistics regarding previous beam switches and/or measurements. The candidate beams for a serving beam may, for example, comprise beams that have been used before and/or after a beam switch to/from the serving beam, beams that have been associated with strong reference signal measurements for the serving beam, and/or beams adjacent to the serving beam and supported by the serving network node. The candidate beams for a geographical position may, for example, comprise beams that have been associated with strong reference signal measurements for the geographical position, and/or any combination with information regarding serving beam. The database may, additionally, comprise (average) signal levels of the reference signals for some of the candidate beams (e.g. the strongest) based on earlier measurements for each serving beam and/or geographical position.
One problem with beam-forming systems (especially systems with narrow beams) is that in some situations, the signal power (and typically the signal-to-interference ratio) may decrease a lot during a very short time span. This time span may be so short that there is not enough time to determine that it is probable that a beam switch is needed, to trigger a mobility procedure and to complete a beam switch. Thus, the connection between the wireless communication device and the network may be lost (e.g. due to out-of-sync and subsequent radio link failure).
FIG. 1 illustrates an example scenario where a sudden drop of the signal strength may be experienced such that there is not enough time to prepare for and carry out a beam switch as needed.
In FIG. 1, a wireless communication device has an ongoing connection with network node 120 via beam 110 when the wireless communication device is in position 100a. When the wireless communication device moves around a corner of a building 130 it ends up in a new position 100b where signals of the beam 110 cannot reach it (or reaches it with a very low signal level) due to shadowing by the building 130. Furthermore, interfering beams 111 (from network node 120, reflected of the building 131) and 112 (from network node 121) may be received at high signal levels by the wireless communication device in position 100b, which results in a low signal-to-interference ratio.
Since the process of moving around the corner may be fast, the signal power of beam 110 (and the signal-to-interference ratio) may drop very quickly and the connection between the wireless communication device and the network may be lost as explained above.
Thus, there is a need for improved (or at least alternative) approaches to mobility in cellular communication systems employing beam-forming.